1. Field of the Invention
This invention relates to a phase change optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change of reflectance by the crystallographic change is detected for reproduction of the information. The phase change optical recording media are of great interest since the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of phase change type used chalcogenide systems such as Ge--Te system and Ge--Sb--Te system which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was also recently proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds have been investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228. Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai Nos. (JP-A) 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992. In addition to the optical recording media of phase change type wherein chalcopyrite compounds are used, optical recording media of phase change type wherein AgSbTe.sub.2 phase is formed with the crystallization of the recording layer is disclosed in JP-A 267192/1992, 232779/1992, and 166268/1994.
In the optical recording media of phase change type, a groove is formed in the substrate for the purpose of tracking, and address information is sometimes loaded in the groove. It has been common to form record marks in the groove, or in the region (land) formed between the adjacent grooves. Recently, land/groove recording has been proposed wherein both the land and the groove are used for the recording tracks to thereby reduce the track pitch and enable a high density recording (Japanese Patent Publication No. (JP-B) 57859/1988).
When the recording track pitch is reduced as in the case of the land/groove recording, spot of the laser beam used for the recording/reproduction will extend into the adjacent track to result in an increased crosstalk during the reproduction.
JP-A 282705/1993 proposes an optical recording medium wherein width of the land and width of the groove are substantially equal, and the depth of the groove (light path length) is adjusted to 1/7 to 5/14 of the recording/reproducing wavelength to thereby reduce crosstalk in the land/groove recording. JP-A 134838/1995 proposes an optical recording medium wherein width of the land and the groove are substantially equal and the depth of the groove is adjusted as in the case of the JP-A 282705/1993, and wherein relation between the thickness of the second dielectric layer on the recording layer and the thickness of the reflective layer on the second dielectric layer is limited to ensure consistent recording characteristics between the land and the groove.
However, the limitation of groove depth as described in JP-A 282705/1993 and JP-A 134838/1995, supra, is associated with the decrease in output of the push-pull signal for tracking. The output of the push-pull signal is highest when the groove depth (light path length) is 1/8 of the recording/reproducing wavelength, and drastically reduces with the increase in the groove depth. When the groove depth is 1/7 to 5/14 of the recording/reproducing wavelength, tracking signal of the intensity required in mass-storage, high transfer rate optical recording medium is not obtained, and operation will be unstable.
The optical recording medium of phase change type utilizes difference in reflectance between the crystalline and the amorphous state, and light absorption (Ac) of the recording layer in the region other than record marks (in crystalline state) and the light absorption (Aa) of the recording layer in the record marks (in amorphous state) are often different, and the Ac&lt;Aa is the condition generally found in such situation. It should be noted that Ac and Aa are values measured at the wavelength of the recording/reproducing laser beam. Recording sensitivity and erasability are thus different depending on whether the region overwritten is crystalline or amorphous, and consequently, the record marks of different length and width are formed by the overwriting to invite increase in the jitter often resulting in errors. When mark edge recording wherein the information is encoded in opposite edges of the record marks is adopted for increasing the recording density, variation in the length of the record marks has greater significance and such variation invites increased errors. In order to solve such situation, it is preferable that the difference between Ac and Aa is small, and more preferable that Ac&gt;Aa in consideration of the latent heat. The relation of Ac&gt;Aa may be realized by regulating the thickness of the recording layer or the dielectric layers sandwiching the recording layer. However, in the medium of conventional structure, the relation of Ac&gt;Aa results in reduced difference between the reflectance (Rc) of the medium of the region other than the record marks and the reflectance (Ra) of the medium in the record marks, and hence, in a reduced C/N.
In view of such situation, JP-A 124218/1996 proposes an optical information recording medium comprising a substrate, a first dielectric layer, a recording layer, a second dielectric layer, a reflective layer, a third dielectric layer, and a UV cured resin layer disposed in this order wherein Ac&gt;Aa, and an extremely thin metal film of high light transmission, Si or Ge is used for the reflective layer, and a dielectric material having a refractive index of higher than 1.5 is used for the third dielectric layer. The relation Ac&gt;Aa is achieved without detracting from the high (Rc-Ra) by providing the reflective layer of high light transmission and the third dielectric layer of high refractive index. The structure wherein the difference between Ac and Aa is small or the structure wherein Ac&gt;Aa is herein referred to as absorption coefficient control structure.
As summarized above, no effective suggestion has been made for ensuring both the tracking signal output of sufficient level and reduced crosstalk in an optical recording medium of absorption coefficient control structure.